1. Field of the Invention
The invention relates generally to a method of fabricating flash memory devices and, more specifically, to a method of fabricating flash memory devices, in which oxidization of a tungsten hard mask film can be prevented and bit line capacitance can be reduced.
2. Discussion of Related Technology
70 nm-grade NAND flash devices are currently fabricated as follows. Gate and source/drain junctions are first formed, and a first interlayer insulation film is formed on the entire surface of a substrate. A source contact that is electrically connected to the source junction through the first interlayer insulation film is formed, and a second interlayer insulation film is formed on the entire surface. A drain contact that is electrically connected to the drain junction through the first and second interlayer insulation films is formed, and a stop nitride film and an oxide film are formed on the entire surface. The oxide film and the stop nitride film are etched to form a trench through which the second interlayer insulation film on the drain contact and the source contact are exposed. A contact hole through which the source contact is formed in the second interlayer insulation film is exposed through the formation of the trench. A cleaning process is then performed and a metal material such as tungsten (W) is buried into the trench and the contact hole to form a bit line connected to the drain contact and a source line connected to the source contact.
In such a scheme, the oxide film is lost due to the cleaning process. A wet barrier is added to the lateral sides of the trench in order to prevent loss of the oxide film.
Meanwhile, to prevent defective patterns of the oxide film due to damage of a photoresist (PR) used to etch the trench, a tungsten hard mask film is used. The tungsten hard mask film partially remains even after the trench is etched.
In the case where an oxide film by a typical low-pressure chemical vapor deposition (LPCVD) method is used as the wet barrier, however, a problem arises because a residual tungsten hard mask film is oxidized. To prevent this problem, a nitride film is used as the wet barrier instead of the oxide film.
Since the nitride film has a dielectric constant higher than that of the oxide film, however, problems arise because bit line capacitance increases and the bit line speed is reduced.